1. Field of the Invention
The present invention relates to a tire for vehicle wheels, reinforced with metallic cords.
The invention is not restricted to a specific type of tire, but is particularly convenient when applied to tires for two-wheeled vehicles, because of their different behaviour during cornering.
2. Description of the Related Art
When travelling along a curved path, two-wheeled vehicles lean towards the inside of the curve at an angle, called “camber” angle, which can be as much as 65° to a plane which is vertical with respect to the ground; this manoeuvre gives rise to a (camber) thrust in the tires which opposes the centrifugal force acting on the vehicle.
As the velocity increases and the radius of curvature of the path decreases, the inclination of the vehicle alone is no longer sufficient to compensate for the effect of centrifugal force: it is therefore necessary to increase the thrust, and this is achieved by inclining the plane of rotation of the front tire with respect to the tangential direction of the curved path at an angle called the “slip angle”, by a manoeuvre performed by the rider by means of the handlebars.
Thus the necessary increase in the slip thrust is obtained.
The rear tire, as is well known, is non-steering, and provides a smaller and qualitatively different thrust, namely one of the linear type, as opposed the curvilinear thrust of the front tire: the mutual combination of these two thrusts determines the attitude of the moving vehicle.
Since a two-wheeled vehicle is less stable than a four-wheeled vehicle, this specific behaviour of a two-wheeled vehicle makes it necessary for the vehicle to have better roadholding and superior manoeuvrability for the riding safety of the driver. In particular, it makes it necessary for the tires to offer high performance in terms of roadholding, stability of running, continuity and progressiveness of variation of the forces applied to the vehicle and exchanged between the tire and road.
The applicant has perceived that an increase in the velocity developed by the vehicle is accompanied by an increase in the importance of a further requirement, hereafter referred to as the “contact feeling”, which can be defined as the reassuring sensation of managing of the vehicle transmitted by the tire to the rider, who thus becomes certain that the vehicle can adhere strictly to the required trajectory, without skips or “lifting” or any undesired movements, in all running conditions.
The “contact feeling” is closely related to the compliance of the tire to the footprint, intended as the capacity of the tire to adapt itself to the ground.
This requirement, which has not been perceived or given due consideration in the past, since it is of little significance at low velocities, is not adequately met by the tires available at present. One object of the present invention is therefore to provide tires, particularly for fitting to two-wheeled vehicles, which can transmit a high contact feeling to the rider.
The modern technology for manufacturing high-performance motorcycles, serially-produced models of which can achieve speeds of more than 300 km/hr, requires, in order to achieve this performance, not only suitable propulsions, but also a reduction of the total weight of the vehicle, attainable by making the tyres lighter, too.
The reduction of the weight of a tire has a further advantage. The tire is one of the so-called non-suspended masses, i.e. one of the parts which move as the wheels move, together with, for example, the rims, suspensions, brake discs and transmission chains. The reduction of the weight of the non-suspended masses not only makes the vehicle lighter but also improves its driving characteristics.
A further object of the present invention is therefore to provide tires, particularly for fitting to two-wheeled vehicles, which are lighter than current tires but have equal performance.
One way of decreasing the weight of a tire is to lighten the metallic reinforcing cords; in tires for two-wheeled vehicles, this approach is particularly advantageous, since the metallic cords of the ply are the heaviest part of the structural elements of the tire, and are located on its greatest circumference. A reduction of their weight not only decreases the weight of the tire, but also reduces its inertial moment, and consequently the behaviour of the tire improves both because of the reduction of the weight itself and because of the position where this reduction is achieved.
The use of metallic cords in reinforcing structures of tires is now well known and does not require detailed explanation; in any case, information useful for the understanding of the invention will be provided in the remainder of this description.
A metallic cord generally consists of a plurality of elementary filaments wound together in various geometrical configurations, made stable by means of a suitable permanent deformation, of the bending type at least, carried out on the aforesaid filaments, and by means of the said geometrical configuration.
The metallic cords designed for reinforcing articles having an elastomeric matrix, such as tires, must have characteristics of high penetrability of rubber among the constituent filaments, and of considerable structural strength to provide good performance by the article in use.
At the present time, some types of tires produced by the Applicant use a belt system consisting of a spiral arrangement of at least one metallic cord. The cord consists of strands which in turn consist of elementary filaments. For example, the 3×4×0.20 HE HT cord is a cord consisting of three strands, each consisting of four filaments having a diameter of 0.20 mm each.
This type of cord allows good penetration of rubber among the strands, but low, and sometimes zero, penetration of rubber into the interior of each strand.
The symbol HE means “High Elongation” and indicates that the winding of the filaments in the strands and of the strands in the cord is done in the same direction (the so-called Lang's lay structure). Additionally, the winding is such that it allows small movements of the strands with respect to each other. Thus the finished cord essentially acquires its own specific load-elongation diagram which shows a significant change of slope (knee) in the values of elongation when a particular load is reached. In particular, the initial portion of the diagram shows very consistent elongations for small variations of load, and is connected, via the knee, to the final portion of the diagram which shows very small elongations for consistent variations of load. A cord of this type is described in EP-B-461 646 in the name of the Applicant. The mid-knee value determines the correct assembly and moulding of the tire. For this purpose, it is preferably in the range from 0.4% to 1% of the elongation. This type of cord has proved its usefulness for moulding tires with radial carcass, particularly motorcycle tires.
The symbol HT means “High Tensile” and indicates a very small breaking load, for the filaments of the cord, of about 850 N (measured by the method of BISFA-E6 “Breaking load and elongation at break”, BISFA-“Internationally agreed methods for testing steel tyre cord”, Ed. BISFA, 4th Avenue E. Van Nieuwenhuyse, B1160 Brussels, Belgium). This is dependent, inter alia, on the percentage of carbon in the st el forming the filaments of these cords; in the case in question, this percentage is greater than 0.8%.
These cords are particularly applicable, for example, as reinforcing elements in the radially outermost belt layer of certain tires where they are laid at a substantially null angle of (0°) to the circumferential direction of the tire.
Recently, cords in which at least one of the constituent filaments is deformed by angular waving (“crimped”) or by sinusoidal waving have been proposed for use in tires. An example is described in patent application WO 95/16816 (in the name of N. V. Bekaert S. A.), according to which the filaments are stranded together in a loose way so that the penetrability of the cord is increased, and are preferably waved with a polygonal rather than sinusoidal configuration.
In this context, WO 99/28547 (in the name of N. V. Bekaert S. A.) relates to a cord of the type described above, characterized in that at least one of the constituent filaments is provided with a first crimp in one plane and a second crimp in a plane substantially different from the plane of the first crimp. These cords, particularly in the single-strand version, have no substantial links among the constituent filaments, and are geometrically irregular, non-uniform and unstable.
WO 00/26465, in the name of the Applicant, claims cords characterized in that the filaments show a deformation by sinusoidal waving in a single plane.
The term “deformation by sinusoidal waving” denotes that the filament is deformed uniformly with a wavelike pattern without sharp angles in the three spatial planes.
In order to reduce the weight of a cord, it is necessary to reduce the diameter and/or the number of the filaments, but this implies a reduction of the breaking load of the cord. For a 40% decrease in weight, an HE HT cord shows an substantially equivalent reduction of the breaking load. This reduction of the breaking load is unacceptable in tire technology, in which the tire structure has its constituent materials (mixtures and reinforcing elements) located only where they are useful and only in the quantities which are strictly necessary for the purpose which they serve.
Moreover, the reduction in the filament diameter makes it impossible in practice to produce a cord with an acceptable “knee” to permit the processing and, especially, the moulding of the tire: this is due to the fact that, as the filament diameter decreases, it becomes more difficult to impart, during the cord production process, the permanent deformations which determine the final properties of the cord.
When faced with the above problem, a person skilled in the art knows that it is not possible to modify the known HE HT cords, because a reduction in weight due to the reduction in the diameter and/or the number of the filaments, as stated above, compromises the ability to process the tire and in any case decreases the safety factor of the product below the acceptable limits.
The use of cords with waved filaments as described above appears to be difficult to envisage, since the irregular, non-uniform and unstable geometry due to their structural characteristics, and the corresponding load-elongation diagram, do not appear to be capable of meeting the conditions for processing and practical use of the tire.